Abstract

Biological stability is one of the most important aspects for drinking water safety. The presence of organic matters in finished water would promote bacterial regrowth that in turn would lead to corrosion of pipe materials and deterioration of water quality in distribution systems(DSs). Assimilable organic carbon(AOC) and biodegradable dissolved organic carbon(BDOC) have been widely considered as indexes which can measure biological stability and bacterial regrowth potential of drinking water. When concentrations of AOC and BDOC are controlled under thresholds, biological stable drinking water which does not support bacterial regrowth will be prepared. Thus, study on removal of organic matters and biological stability of drinking water are focuses of water-treatment field in recent years. In this paper, the methods to measure AOC and BDOC are improved. Removal capability of organic matters by water purification processes and changes of biological stability in DSs are investigated,. Two advanced and enhanced processes are also explored. Main contents and results are summarized as follows:1. The growth characteristics of bacterial stains (or inocula) for AOC (or BDOC) bioassay are studied. Different incubation conditions are compared respectively and optimal assay methods are attained. The time need to perform the AOC assay is reduced by increasing the inoculum density (from 500 to 104 CFU/mL) and increasing the incubation temperature (from 15 to 22℃). Yield factors of organisms are measured by an improved method in the lab, P17—1.1×107 CFU/μg acetate carbon and NOX—1.9×107 CFU/μg acetate carbon. The AOC assay can be used in the range of 10～300μg acetate carbon /L. The BDOC method with bacteria attached sand can quickly determine result within 10 days, which is not different from that by the conventional method (with suspended bacteria, 28 days).2. Characteristics and compositions of organic matters from raw water taken at the upper reach of Huangpu Rive are studied by determination of molecular weight(MW) distribution and GC/MS. Removal capability of organic matters by traditional water treatment processes(TWTPs) are investigated in Shanghai, a eastern city of China. The organic matter characters of raw water taken at the upper reach of Huangpu River are high concentrations, excessive species, majority of low MW and affected by seasonal changes obviously. The removal ratios of AOC and BDOC are less than 30 percent by TWTPs. Concentrations of AOC increase dramatically for disinfectant addition. The outputs of water plants used raw water from Huangpu River and Yangzi River are biologically unstable in Shanghai.3. The effects of disinfectants on AOC in drinking water are tested. Chlorine and Chloramine oxidation can increase AOC in drinking water observably. The oxidative reactions of chlorine and Chloramine with organic matters forming AOC have different properties. Changes of biological stability in DSs have been investigated for one years. The results show changes of AOC and BDOC in DSs are attributed to chlorine oxidation and bacterial consuming. And improved models of AOC changes, which are more accordant with the actual conditions of DSs, are proposed based on the law of AOC changes and effects of chlorine oxidation.4. Characteristics of AOC and BDOC are explored by ratios of different organic indexes, distribution of MW and effects on bacteria growth. The relativity between BDOC and DOC is superior to that between AOC and DOC. The ratios of AOC to BDOC for different raw water are diverse, and close for same raw water. AOC is composed of organic matters with low MW mainly. TWTPs can not reduce AOC effectively because removal of low MW organic matters by TWTPs is low. Both AOC and disinfectant residuals are